Abscisic acid (ABA) is a plant hormone that regulates signal transduction associated with abiotic stress responses (Cutler et al., 2010, Abscisic Acid: Emergence of a Core Signaling Network. Annual Review of Plant Biology 61:651-679). The ABA signaling pathway has been exploited to improve plant stress response and associated yield traits via numerous approaches (Yang et al., 2010). The direct application of ABA to plants improves their water use efficiency (Raedmacher et al., 1987); for this reason, the discovery of ABA agonists (Park et al., 2009; Melcher et al., 2010, Identification and mechanism of ABA receptor antagonism. Nature Structural & Molecular Biology 17(9):1102-1110) has received increasing attention, as such molecules may be beneficial for improving crop yield (Notman et al., 2009). The first synthetic ABA agonist identified was the naphthalene sulfonamide named pyrabactin (Park et al., 2009), which efficiently activates ABA signaling in seeds but has limited activity in vegetative tissues, where the most critical aspects of abiotic stress tolerance occur. Sulfonamides highly similar to pyrabactin have been disclosed as ABA agonists (see US Patent Publication No. 20130045952) and abiotic stress modulating compounds (see US Patent Publication No. 20110230350); and non-sulfonamide ABA agonists have also been described (see US Patent Publication Nos. 20130045952 and 20110271408). A complementary approach to activating the ABA pathway involves increasing a plant's sensitivity to ABA via genetic methods. For example, conditional antisense of farnesyl transferase beta subunit gene, which increases a plant's ABA sensitivity, improves yield under moderate drought in both canola and Arabidopsis (Wang et al., 2005). Thus, the manipulation of ABA signaling to improve traits contributing to yield is now well established.
It has recently been discovered that ABA elicits many of its cellular responses by binding to a soluble family of receptors called PYR/PYL proteins. PYR/PYL proteins belong to a large family of ligand-binding proteins named the START superfamily (Iyer et al., 2001); Ponting et al., 1999). These proteins contain a conserved three-dimensional architecture consisting of seven anti-parallel beta sheets, which surround a central alpha helix to form a “helix-grip” motif; together, these structural elements form a ligand-binding pocket for binding ABA or other agonists.